Remotely sensed variability of temperature and chlorophyll in the southern Benguela: upwelling frequency and phytoplankton response

High-resolution (1km) satellite data from the NOAA AVHRR (Advanced Very High Resolution Radiometer) and OrbView-2 SeaWiFS (Sea-viewing Wide Field-of-view Sensor) are used to investigate the upper layer dynamics of the southern Benguela ecosystem in more detailed space and time scales than previously undertaken. A consistent time-series of daily sea surface temperature (SST) and chlorophyll a concentration images is generated for the period July 1998–June 2003, and a quantitative analysis undertaken. The variability in SST, upwelling and phytoplankton biomass is explored for selected biogeographic regions, with particular focus on intra-seasonal time scales. The location and emergence of upwelling cells are clearly identified along the length of the southern Benguela, being distinct on the narrow inner and the mid-continental shelves. Most notable is the rapidly pulsating nature of the upwelling, with intense warm/cold events clearly distinguished. The phytoplankton response to this physical forcing is described. Chlorophyll concentration on the inner shelf largely mirrors the pattern of SST variability, similarly dominated by event-scale processes. Over the mid-shelf, higher chlorophyll is observed throughout all seasons, although low biomass occurs during winter. The variability of the offshore extent of SST and chlorophyll is identified at locations of differing shelf width. Cooler upwelled water is confined primarily to the narrow inner-shelf, with event-scale pulses extending considerable distances offshore. Agulhas Current influences are readily observed, even on the Cape Peninsula inner-shelf. Chlorophyll concentrations vary considerably between the locations of differing shelf width. SST, upwelling and phytoplankton indices are derived for selected locations to quantify the intra-seasonal variations. The SST indices show marked temperature changes associated with rapid pulsation on the event scale. No strong seasonal signal is evident. In contrast, the upwelling indices display a strong seasonal signal, with most intense upwelling occurring in spring/summer in the south. The phytoplankton response to the seasonal upwelling index differs between the selected locations. This study concludes that, although low-resolution SST and chlorophyll data may be useful for investigating general patterns over large scales, higher resolution data are necessary to identify finer scale spatial and temporal variability, especially in the inshore coastal zones.     

Pigment signatures of phytoplankton composition in the northern Benguela ecosystem during spring

Pigment indices were used to investigate the distribution and composition of phytoplankton in the northern Benguela during the austral spring of 2000, with sampling being conducted on five transect lines between 19°S and 25°S and at other inshore stations on the shelf. Total chlorophyll a concentrations (TChla) at the surface ranged from 18.4mg m^?3 at shallow inshore stations to 0.58mg ^m?3 at offshore localities in deeper water. Phytoplankton communities in the inshore environment were characterised by elevated biomass levels and the dominance of diatoms (high Fuc_DP indices). These diatom populations were associated with cool, higher density, nutrient-rich waters near the coast. Phytoplankton biomass declined with increasing distance offshore and the communities changed to a mixed population where small flagellates were generally dominant (high Flag_DP indices). These flagellate communities were associated with warmer, lower density waters. Whereas dinoflagellates (Per_DP Index) and prokaryotes (Zea_DP Index) were observed throughout the region, their contribution to the communities was of secondary importance.     

Diet of sardine Sardinops sagax in the southern Benguela upwelling ecosystem

The diet of sardine Sardinops sagax in the southern Benguela was investigated by microscopic examination of stomach contents. The relative dietary importance of prey size and prey type was assessed by calculating the carbon content of prey items. Sardine is an omnivorous clupeoid, ingesting both phytoplankton and zooplankton, with the relative importance of these two food types varying both spatially and temporally. Stomach contents were numerically dominated by small prey items, principally dinoflagellates, followed by crustacean eggs, cyclopoid copepods, calanoid copepods and diatoms. Virtually all prey items ingested by sardine were <1.2 mm maximum dimension, the particle size below which sardine only filter-feed. Despite the numerical dominance by phytoplankton, zooplankton contributed the major portion to sardine dietary carbon, small calanoid and cyclopoid copepods, anchovy eggs and crustacean eggs being the primary prey types. These results indicate that, like anchovy Engraulis capensis, sardine in the southern Benguela are primarily zoophagous, and contrast with earlier dietary studies on sardine in the region. However, the two species appear to partition their prey on the basis of size; sardine consume small zooplankton, whereas anchovy consume large zooplankton. This difference has been observed in other upwelling ecosystems where the two genera co-exist and is likely to contribute to the regime shifts observed between sardine and anchovy.